Prospero feels betrayed by his brother and it could be argued that his betrayl has masked his judgment of what is right for his people.

The work 'nature' could show how Prospero feels that his brother has always had an evil streek and this act has brought out the evil in him. It could also show that Prospero blames himself for deligating so much power to his brother when he is capable of betrayl.

Prospero usurping Caliban

We learn that Prospero usurped Caliban when he got to the island with Miranda.